The present invention relates to fluid bed apparatus in general and more particularly to an improved refractory grid and fluidizing air nozzle retainer apparatus.
Prior to the present invention, it was common practice to employ refractory grids in fluid bed apparatus for dividing the vessel into an upper material chamber and a lower plenum chamber. The grid also serves to support the bed of material within the material chamber. Refractory materials are most commonly used where high temperature reactions will take place within the fluidized bed of material. As is customary, the grid includes a plurality of spaced apart holes therethrough to permit fluidizing gas to pass from the lower plenum chamber to the upper material chamber for fluidizing material within the material chamber as well as supplying combustion air to the fluid bed where such is required.
It is common practice to place nozzles in the fluidizing air holes in the refractory grid for dispersing the fluidizing gas throughout the fluidized bed. These nozzles also serve to prevent the solid particulate material which makes up the bed of material from sifting through to the plenum chamber. Various techniques have been used for retaining these nozzles in place. One such technique is to merely place the nozzles in the fluidizing air holes but make the nozzles heavy enough so that gravity will hold them in place and the pressure exerted on them by the fluidizing air will not cause them to be blown out of the air holes into the material chamber. As long as conditions are as designed with steady reactor operation, the nozzles will remain in place. However,if there is a sudden increase in plenum chamber pressure or decrease in material chamber pressure, the weighted nozzle can be blown out of the grid leaving the fluidizing air hole open and permitting the bed to pour down this now open fluidizing air hole into the plenum chamber. It is, of course, undesirable to have the fine material in the plenum chamber because this material can be valuable product which will be lost, or it can be abrasive, swirl around in the plenum and cause wear on the bottom of the grid or the plenum walls, or the fine material can pass up through other nozzles causing excessive wear on these nozzles. Economics is a further disadvantage of making the nozzles heavy to withstand the pressure of air. Fluidizing nozzles for high temperature fluid bed apparatus are usually made from costly corrosion and temperature resistant materials. The heavier the nozzle, the more costly it is to produce.
Other methods are used for retaining nozzles in place. These include the use of enlarged holes and tapered nozzles as shown in U.S. Pat. No. 3,672,577. Rods have been attached to nozzles to extend down through the fluidizing air holes with fasteners for securing this rod to the underside of the grid and thus secure the nozzle to the grid. The first arrangement has the disadvantage that the nozzles and grid can be more expensive than other methods. The second arrangement is troublesome where the bottom of the refractory is subjected to wear caused by fine abrasive material which may enter the plenum chamber. In addition, thermal expansion causes problems when a tie rod arrangement is used. When the tie rods become hot, they expand. As a result the fasteners move away from the under surface of the grid and the nozzles are no longer held firmly in place.
Typical refractory grids for fluid bed apparatus are subject to wear problems. The constant swirling motion of the particles which make up the fluid bed abrade the surface of the grid. The abrasion is compounded since many fluidizing air nozzles direct the fluidizing air flow downwardly toward the grid surface in order to improve the circulation of air and material through the bed. As a result, the abrasive material wears away the surface of the grid. After a period of operation this wear can become so severe that the grid must be replaced. It is therefore important to improve refractory grid life.
By the present invention, apparatus has been found which will serve to not only improve the wear life of a refractory grid, but also serve to provide a means for holding fluidizing air nozzles in place.